Electric Current Tuning the Self-Oscillation Frequency of EC- VCSELs. IEEE Photonics Technology Letters, 2013, v. 25 n. 17, p.

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1 Title Electric Current Tuning the Self-Oscillation Frequency of EC- VCSELs Author(s) Smith, C; Li, W; Wysocki, G; Chou, S Citation IEEE Photonics Technology Letters, 2013, v. 25 n. 17, p Issued Date 2013 URL Rights IEEE Photonics Technology Letters. Copyright IEEE.

2 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 17, SEPTEMBER 1, Electric Current Tuning the Self-Oscillation Frequency of EC-VCSELs Clinton J. Smith, Student Member, IEEE, Wen-Di Li, Gerard Wysocki, and Stephen Y. Chou, Fellow, IEEE Abstract We demonstrate a new way to electrically tune the self-oscillation frequency of an external cavity VCSEL by changing the electrical current injected into the VCSEL, without using any mechanical moving parts. We found that for a selfoscillation frequency from 500 MHz to 4 GHz, the tuning range is up to 15% and the tuning rate is up to 800 MHz/mA. Our experiments and simulation show that the injection current tuning of self-oscillation frequency is due to a change in the VCSEL birefringence induced by changing the injection current. Index Terms Lasers and electrooptics, electrooptic effects, vertical cavity surface emitting lasers, birefringence, laser applications, optical modulation. I. INTRODUCTION HIGH frequency modulation of laser beams has many applications, such as in optical communications networks or accurate time-keeping devices (e.g., ultra-low power frequency modulation (FM) sidebands for optical atomic clock applications) [1] [7]. One important challenge in EC-VCSELs is to tune the oscillation frequency. An EC-VCSEL consists of a VCSEL, a quarter-wave plate (QWP) with 45 rotation from the VCSEL polarization axes, and a partial reflector (PR) (Figure 1). Traditionally, the frequency tuning is achieved by mechanically changing the cavity length (moving the PR) or rotating the QWP orientation [1], [3], [8] which requires bulky mechanical stages or piezo-actuators. However, in many applications, such mechanical tuning is undesirable, due to tuning inaccuracy, bulky size, tuning speed, and large power consumption. There is a great need as well as a great advantage to have an electrical method for frequency tuning because it would offer much better tuning accuracy and would not involve any mechanical moving parts. Here we report a new method and its experimental demonstration of electrically tuning the EC-VCSEL oscillation frequency by changing the electrical injection (drive)-current Manuscript received May 14, 2013; revised June 10, 2013; accepted July 6, Date of publication July 11, 2013; date of current version August 6, This work was supported in part by DARPA, the National Science Foundation s MIRTHE Engineering Research Center under Grant EEC , and the National Science Foundation under Grant Nanotechnology for Clean Energy IGERT. C. J. Smith, G. Wysocki, and S. Y. Chou are with the Department of Electrical Engineering, Princeton University, Princeton, NJ USA ( chou@princeton.edu). W.-D. Li is with the Department of Electrical Engineering, Princeton University, Princeton, NJ USA, and also with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong Color versions of one or more of the figures in this letter are available online at Digital Object Identifier /LPT IEEE Fig. 1. (a) Schematic showing a typical external cavity configuration used for self-modulation (QWP = quarter-wave plate, PR = partial reflector, POL = polarizer). (b) Schematic perspective along the optical axis showing rotational orientation of QWP s neutral axes at a 45 offset relative to the VCSEL s polarization axes. without using any mechanical moving parts. The accuracy of the EC-VCSEL oscillation frequency is only limited by the resolution and stability of the current source and the mechanical stability of the external cavity. We have demonstrated EC- VCSELs with self-oscillation frequencies from 500 MHz to 4 GHz, a frequency tuning range up to 15%, and a tuning rate up to 800 MHz/mA. Our experiments and simulations show that the drive current tuning of the self-oscillation frequency is due to a current induced change in the VCSEL birefringence. II. TUNABLE EC-VCSEL STRUCTURE AND EXPERIMENTAL SETUP The tunable EC-VCSEL we built consists of an 850 nm wavelength VCSEL chip (Avalon Photonics AVAP-850SM), a collimating lens, a thin QWP, and a PR (Fig. 1). The VCSEL chip was mounted on an aluminum block set on a 5-axis stage (Newport), and was driven by a laser current driver (Newport model 5005). For testing the EC-VCSEL output, another QWP, a polarizer (POL) and a fast photodetector (New Focus 1577-A 12 GHz) are placed in the path of the beam emitted from the EC-VCSEL. The photodetector was connected to an RF spectrum analyzer (HP8569B) to gather frequency spectra. Two types of QWP s are used. The first type is a nanoimprinted subwavelength optical element consisting of an amorphous silicon grating of 200 nm pitch on top of a fused silica substrate, which is a zero order QWP [2]. The second type of QWP is a Newport multi-order quartz piece. Both give λ/4 birefringence at 850 nm wavelength.

3 1708 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 17, SEPTEMBER 1, 2013 The PR consists of 4 nm thick Au which was thermally evaporated by e-beam onto the back side of each QWP. The PR has approximately 12% reflection, 38% transmission, and 50% absorption. Of the 12% reflection, 4% at most is coupled back into the VCSEL cavity (approximately Chraplyvy s 3rd or 4th region) [9]. The combination QWP-and-PR element was mounted on a Newport 5-axis stage, and its position was adjusted to control the cavity length, L, to set the selfmodulation center frequency ( f c = c/4l). III. EXPERIMENTAL RESULTS A. Tuning Frequency by Changing Drive Current For a cavity length of 1.2 cm, the self-oscillation frequency of the EC-VCSEL was 4.35 GHz at a drive current of 6.25 ma. When the drive current was increased to 6.80 ma, the frequency was increased to 4.75 GHz, giving a tuning rate of 727 MHz/mA (Fig. 2(a)). The self-oscillation frequency stability was observed to be 8 MHz, and was found to be related to the accuracy of the laser current source (7.6 μa), which would give an expected 5.5 MHz frequency variation. As the self-modulation frequency is tuned far away from the center frequency by changing the drive current, the quality of the self-modulation signal degrades: The full-width-athalf-maximum (FWHM) of the self-oscillation frequency is observed to increase while the oscillation amplitude decreases. For instance, at 4.35 GHz the FWHM is 41.6 MHz with 84.2 dbm oscillation amplitude, but at 4.75 GHz the FWHM is 91.5 MHz with 88 dbm oscillation amplitude. Similar behavior has been observed in other external cavity systems we tested. Such degradation is attributed to the fact that the output intensity as a function of optical frequency depends on the gain curves [10], [11]. If the drive current is further increased, the signal is found to disappear into the background noise, namely the EC-VCSEL ceases to oscillate. The drive-current based frequency tuning was also observed for the EC-VCSELs of longer or shorter external cavity lengths (Fig. 2(b)), such as from 14.8 cm to 1.55 cm (i.e., the oscillation frequency of 500 MHz to 4 GHz) [12]. The frequency tuning range of the EC-VCSELs by the drive current was found to be 3 15% of their self-modulation frequency, and depends on the external cavity length. The longer the cavity length, the wider tuning range is. For example, the frequency tuning range is 15% over 0.5 ma drive current change for the 14.8 cm cavity; and 4.5% over 2.5 ma drive current change for the 1.55 cm cavity (Fig. 2(b)). B. Tuning Frequency by Rotating Quarter-Wave Plate (QWP) As discussed later, theory also predicts the center frequency can be tuned by rotating the QWP rotational angle. Experimentally, in an EC-VCSEL of 4.4 cm length cavity, an 8 QWP rotation offset from 45 results in a 90 MHz self-modulation frequency shift from 1.61 GHz to 1.70 GHz (Fig. 3). Similarly as in drive-current tuning, the self-oscillation signal is degraded as the QWP rotation angle is increasingly offset away from 45. Fig. 2. (a) The EC-VCSEL (1.2 cm cavity length) self-modulation frequency is tuned by the VCSEL drive current. (black) 4.35 GHz self-modulation frequency with 6.25 ma drive current; (red) 4.75 GHz self-modulation frequency with 6.80 ma drive current. (b) Normalized self-modulation frequency tuning from a starting frequency for a variety of EC-VCSEL cavity lengths. Each plot shows the drive current at which the self-modulation signal is observed (x-axis) and the fraction by which the self-modulation frequency increases as a function of drive current (y-axis). IV. THEORY: CAUSE OF THE CURRENT TUNING The electrical drive current tuning of the EC-VCSEL oscillation frequency can be attributed to the drive current induced change in the VCSEL birefringence, which, in turn, affects the frequencies of the orthogonal modes of the external cavity. It is shown theoretically that the VCSEL birefringence (radians), δ, and oscillation frequency, f, depend on the electric-field which is a function of the electrical current (van Exter et al., [13] and Ginovart et al., [8]): n 3 δ r 41 E DC (1) n gr f = c ( 1 ± 2 ( )) 1 4L π arctan r1 r 2 (2) 2δ cos(2) where n is the spatially averaged refractive index in the presence of zero electric field, n gr is the spatially averaged group refractive index, r 41 is the spatially averaged electrooptic coefficient, E DC is the spatially averaged electric field weighted by the local optical intensity, L is the length (cm) of the external cavity, r 1 and r 2 are the VCSEL facet reflectivities, and is the rotational angle of the QWP with respect to the polarization axes of the VCSEL.

4 SMITH et al.: ELECTRIC CURRENT TUNING OF SELF-OSCILLATION FREQUENCY OF EC-VCSELs 1709 Fig. 3. (a) Calculated self-oscillation frequency for a given QWP rotational angle in relation to the VCSEL polarization axes. (b) Rotation of the external cavity QWP by 8 results in a self-modulation frequency shift from 1.61 GHz to 1.70 GHz. Calculated results in (a) agree with experimental results in (b). A change in VCSEL drive current causes a change in electric field, leading to birefringence changes (Eqn. 1) and hence the EC-VCSEL s frequency change (Eqn. 2). In fact, experimentally it has been observed that the VCSEL birefringence changes with increasing drive current/electric field [14]. V. COMPARISON OF EXPERIMENTS WITH THEORY We compared our experiments with the theory of the EC-VCSEL s self-modulation frequency tuning. Birefringence values are given both in GHz relative to the optical frequency at THz (850 nm wavelength) and in radians. If we assume r 1 (100%), r 2 ( 99%), δ = 20 GHz (0.356 mrad) and a 4.4 cm length cavity, Eqn. 2 gives the self-modulation frequency of 1.65 GHz for 45 QWP rotation and 1.74 GHz for a QWP rotation of 35, leading to 90 MHz oscillation increase over 10 QWP rotation change (Fig. 3(a)), which is consistent with our experiments (Fig. 3(b)). Next, using r 1 and r 2 above and a 1.2 cm external cavity (Fig. 4(a)), the EC-VCSEL self-modulation frequency was calculated as a function of both δ and QWP rotational angles using Eqn. 2. The calculation shows that the EC-VCSEL self-modulation frequency increased with increasing VCSEL birefringence. The same degree of self-modulation frequency tuning (400 MHz) has been experimentally observed (Fig. 4(b)) in an Fig. 4. The (a) calculated self-oscillation frequency for a 1.2 cm external cavity system as a function of VCSEL birefringence and QWP rotational position; and the experimental results (b) for the same system. EC-VCSEL of the same cavity length (1.2 cm) and QWP offset (36 38 ) as shown in the simulations based on Eqn. 2. This tuning value corresponds to a VCSEL birefringence change of about 55 GHz (0.979 mrad). VI. CONCLUSION We demonstrated a new way to electrically tune the selfoscillation frequency of EC-VCSELs, without using any moving parts. By changing the electrical current injected into the VCSEL, EC-VCSELs with self-oscillation frequencies centered from 500 MHz to 4 GHz were shown to tune their frequencies up to 15% with a tuning rate up to 800 MHz/mA. Our experiments compared with simulations of the EC-VCSEL self-modulation frequency as a function of QWP offset angle and VCSEL birefringence show that the self-oscillation frequency change is due to a change in the VCSEL birefringence, which is dependent on the VCSEL injection current. ACKNOWLEDGMENT S. Y. Chou originated, designed, and directed the research. C. J. Smith, W. D. Li, and S. Y. Chou designed and performed the experiments. C. J. Smith, S. Y. Chou, and G. Wysocki contributed to data analysis. C. J. Smith, S. Y. Chou, and G. Wysocki wrote the manuscript.

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